This invention relates to compositions classified in the art of chemistry as cellulose esters and to processes for their preparation from dissolving pulp derived from lignocellulosic materials such as wood pulp.
The fibers of freshly bleached dissolving pulp while still in wet sheet form prior to entering the dryer section of a paper machine are in a highly swollen state. If they were to be used in an esterification reaction in this condition their reactivity would be quite high. Unfortunately, the nature of the dissolving pulp and the cellulose ester business requires that the pulp be dried by the pulp producer for storage and shipment to the processor who will esterify the pulp.
During the drying process shrinkage forces cause a partially irreversible collapse of the structure of the cellulose fibers. This effect, which is known as hornification, is especially pronounced in the case of dissolving pulps which have been post treated with cold alkali to remove hemicelluloses. As a result of hornification, pulp reactivity in chemical processes, in which reagents are unable to effectively swell the fibers of the pulp to reach the chemically reactive sites, is severely reduced. One such process is the process of cellulose esterification as it is practiced industrially.
The use of cold alkali purified dissolving pulps in esterification processes requires the use of harsher reaction conditions involving higher temperatures and/or longer reaction time in the activation step. Because of the constraints in their design and the expense of modifications such harsher conditions are not feasible in many esterification plants. As such harsher conditions are, obviously, a source of additional expense even for plants which are capable of providing the requisite equipment for their use, any modification of cold alkali purified dissolving pulp which would permit its activation for esterification under milder conditions would be of benefit to all users of the pulp for manufacture of cellulose esters and in addition would provide a broader base source of supply for those plants presently incapable of utilizing this type of pulp to maximum advantage in their operations.